NASA attempts to rescue planet-hunting probe

By Matt Smith, CNN

Updated 2225 GMT (0525 HKT) July 18, 2013

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Where life might live beyond Earth – This diagram lines up planets recently discovered by Kepler in terms of their sizes, compared to Earth. Kepler-22b was announced in December 2011; the three Super-Earths were announced April 18, 2013. All of them could potentially host life, but we do not yet know anything definitive about their compositions or atmosphere.

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Where life might live beyond Earth – This illustration depicts Kepler-62e, a planet in the habitable zone of a star smaller and cooler than the sun. It is located about 1,200 light-years from Earth in the constellation Lyra.

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Where life might live beyond Earth – This illustration depicts Kepler 62f, a planet in the habitable zone of a star smaller and cooler than the sun, in the same system as Kepler 62e.

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Where life might live beyond Earth – This diagram compares the planets of our own inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth. Kepler-62e and Kepler-62f are thought capable of hosting life.

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Where life might live beyond Earth – The planet Kepler-69c is located about 2,700 light-years from Earth in the constellation Cygnus. This is an illustration of the planet, which is the smallest yet found to orbit in the habitable zone of a sun-like star.

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Where life might live beyond Earth – This diagram compares the planets of our own inner solar system to Kepler-69, which hosts a planet Kepler-69c that appears to be capable of hosting life, in addition to planet Kepler-69b.

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Where life might live beyond Earth – This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft.

Stargazers crossed their fingers Thursday as NASA attempted to revive the planet-hunting Kepler probe, idled since a piece of critical equipment gave out in orbit two months ago.

Kepler has been sidelined since mid-May, when a reaction wheel that helps aim the spacecraft's telescope failed. Controllers launched a series of tests that will determine whether that device can be restarted, or whether another reaction wheel that quit in July 2012 can be reactivated.

Controllers have remained in communication with the craft, which is about 45 million miles from Earth. It takes about four minutes for a radio signal to traverse that gap.

In the first round of tests, telemetry from the craft indicated the wheel that shut down in May spun counterclockwise, but didn't respond to commands to turn clockwise, NASA reported. Controllers will go ahead with tests scheduled for next week on the other wheel while they study the results of Thursday's effort.

"While this is a positive start, it is very early in the multi-stepped process to characterize the performance of the reaction wheels and to determine if one could return to operation," Hunter said in a written statement. "The team will remain focused on the upcoming tests and report the cumulative test results at the end of the month."

Kepler was built with four reaction wheels and needs three to keep the spacecraft aimed precisely at its targets -- one to fine-tune the spacecraft's position on each axis. After May's failure, only two remained in service and the probe went into a low-powered state to preserve fuel.

"We're kind of hoping for the best, but planning for the worst," said Sara Seager, an astrophysicist at the Massachusetts Institute of Technology.

Kepler's observations have focused on a sliver of the cosmos around the Northern Hemisphere constellations Cygnus and Lyra. They have led scientists to believe that most stars in our galaxy have planets circling them.

The craft's light-recording photometer has shown that the intensities of as many as two-thirds of stars are more variable than our sun.

And in April, researchers reported that they had identified three planets that could potentially host life. Two of those -- Kepler 62e and 62f, located about 1,200 light-years away -- are considered the best candidates so far.

"We really want Kepler to be able to return to be able to operation in the same way that it was before the wheel failed," said Seager, a former member of the Kepler science team. "That's the ultimate hope."

But the probe has already surpassed its three-and-a-half-year minimum expected lifespan, and it has collected enough data to keep scientists busy for up to two years. And even if Kepler can't regain the kind of steady precision required for its observations to date, NASA may be able to use the spacecraft for other purposes, Seager said.